The present invention relates to a combination hydraulic and mechanical steering system, and more particularly to such a steering system for a vehicle having multiple independently steerable wheel sets, and most particularly, to a cam assembly for translating the motion of an operator manipulated steering wheel into linear motion for controlling a servomechanism that angularly positions each of the wheel sets in a plurality of different steering modes.
Vehicles used for transporting large loads over relatively small distances, such as from a fabricating section of a shipyard to the assembly section of a shipyard, normally have a plurality of wheel sets supporting a load-bearing platform. Since the loads imposed upon such transporting vehicles are relatively large, the vehicles require a large number of wheels to distribute the load to the ground. The wheel sets are usually differentially steerable to provide adequate maneuvering capability and to eliminate tire scuffing when the vehicle is turned. Prior art transporting vehicles employ steerable wheel sets that provide the capability to turn in a circle whose center is centrally and laterally offset relative to the longitudinal dimension of the transporting vehicle. This mode of steering is normally referred to as the conventional mode. Since the plurality of wheel sets are distributed longitudinally and laterally under the load platform, each of the wheel sets must have a different turning radius relative to the other so that the turning circles for the wheel sets of the entire vehicle will have a common center. One prior art device for differentially steering the wheel sets of a transporting vehicle employs only mechanical linkages between the various wheel sets. The wheel sets are interconnected by a plurality of tie rods of varying lengths coupled to radius arms affixed to the wheel sets, providing the capability to differentially turn each of the independent wheel sets so that for a given steering command, the turning radius of each of the wheel sets coincides with the common center of the desired turning circle.
In many applications it is desirable to be able to change from the conventional steering mode, wherein the transporting vehicle turns about a common center, to what is known as oblique or "crab" steering, wherein each of the wheel sets is turned to an identical angular position so that the entire vehicle can move transversely to its normally longitudinal direction of travel. With the conventional mechanical linkage used in the prior art, it is impossible to change from the conventional mode of steering to the crab mode of steering without a complete changeover of the mechanical tie rod and turning arm linkages. Because replacement of the mechanical steering linkage in this manner is not economically feasible, present transporting vehicles employing a mechanical steering system are offered with only the conventional steering mode. Some vehicles with mechanical steering linkage can be modified to provide oblique steering; however, all wheels are steered at different angles resulting in unacceptable tire scuffing and mechanical stress on the steering system.
In order to provide the capability of changing from a conventional steering mode to a crab steering mode, each of the wheel sets on the transporting vehicle must have the capability of being steered independently while different means for programming the turning angle of each individual wheel set for a given mode of steering must be provided. In U.S. Pat. No. 3,572,458, issued to Hans Tax, a dual mode steering system for individually steering a plurality of wheel sets is disclosed. This dual mode steering system provides the capability of changing between a conventional steering mode and a crab steering mode, as well as other capabilities. In this system a steering wheel is employed to rotate a shaft carrying a set of cams. Each cam corresponds to a given wheel set on a vehicle and has a follower cam that is coupled to an arm of a potentiometer. A variable voltage, depending upon the position of the arm connected to the cam follower, is transmitted through an electronic control circuit to a servomotor, which in turn drives a rotatable shaft on which a wheel is mounted. Feedback from the shaft to the electronic control circuit is provided via a rotational to rectilinear motion transducer, which in turn drives an arm of a second potentiometer. The variable voltage provided to the electronic control circuit from the second potentiometer is conditioned by the circuit to stop the servomotor at the position predetermined by the position of the arm of the first potentiometer. For the conventional steering mode, a first set of cams is employed that have varying cam surfaces, which are related to the desired turn radius for a given wheel set. A second set of cams, each of which is identical to the other, can be interchanged with the first set of cams to angularly position each of the wheel sets to provide the crab mode of steering. Although the system disclosed by Tax employs a workable means by which the steering mode can be changed, it has the drawback of being electronically controlled. The environment in which many transporting vehicles employing multiple wheel sets are used is not conducive to longevity of the electronic circuitry. For example, a transporting vehicle used in a shipyard is constantly subjected to an influx of dirt and saline water that cause corrosion, which results in maladjustment in or inoperability of the electronic steering circuitry. Thus such systems require constant maintenance and because of their electronic complexity require special repair skills not normally found in vehicle maintenance personnel. Moreover, although the system disclosed by Tax employs a means by which the mode of steering can be changed, that system does not provide a means by which the mode of steering can be quickly changed by an operator from his control station.
It is therefore a broad object of the present invention to provide a steering system for a transporting vehicle having a plurality of steerable wheel sets: that provides the capability to independently steer each of the wheel sets according to a predetermined program; that provides the capability to change quickly among several steering modes, including the conventional steering mode, the crab steering mode, and other modes; that eliminates electronics from the steering control system; that is relatively easy to maintain, that can withstand the adverse environmental conditions, that can be maintained and repaired by one of ordinary skill in vehicle maintenance; that provides a plurality of cam sets, each set corresponding to a given steering mode; and that employs protective devices by which injury to the cam sets or the actuating mechanism can be prevented should part of the overall mechanism become inoperative.